Because of the better coefficient of performance of heat pumps as compared to electrical resistive heating elements, there has been a movement in recent years to purchase electric resistance hot water heaters and then connect the water heater tank to a heat pump so that the heating was actually performed by the heat pump rather than the electric resistance elements in the water heater itself. This produced a good coefficient of performance while at the same time retained the electric resistance heating elements as a standby. One such arrangement is illustrated in U.S. Pat. No. 4,740,673, issued May 26, 1988, entitled "Dual Control Thermostat Circuit" that allows the conventional upper and lower thermostats to control the operation of the external heat pump.
One of the problems associated with heat pump heaters for water heaters is that the heating rate of the heat pump is typically slower than the heating rate associated with the resistance heating elements that come in the water heater. As a result, the heat recovery rate was less than desirable when there was a large drain of hot water from the tank and the upper section of the tank started to cool.